1. Field of the Invention
The present invention relates to a color cathode-ray tube apparatus.
2. Description of Related Art
A color cathode-ray tube apparatus includes a color cathode-ray tube in which an electron gun is housed in an envelope composed of a panel and a funnel connected to each other, and a deflection yoke provided on an outer circumferential surface of the funnel. Three electron beams emitted from the electron gun are deflected in horizontal and vertical directions by the deflection yoke and scan the phosphor screen formed on an inner surface of the panel.
The deflection yoke includes a horizontal deflection coil generating a horizontal deflection magnetic field and a vertical deflection coil generating a vertical deflection magnetic field, and an insulating frame provided between the horizontal deflection coil and the vertical deflection coil. The insulating frame maintains an electrically insulated state between the horizontal deflection coil and the vertical deflection coil, and supports both the deflection coils. On an outer circumferential surface of a substantially cylindrical portion of an end on the electron gun side of the insulating frame, a substantially Ω-shaped metal band is mounted, and both ends of the metal band are fastened with a metal screw, whereby the deflection yoke is fixed to the funnel.
In such a color cathode-ray tube apparatus, in order to enhance an edge of an image to realize high image quality, a velocity modulation coil is used. The velocity modulation coil is composed of a pair of loop-shaped coils attached to positions of the funnel on the electron gun side from the deflection yoke so as to be opposed to each other in a vertical direction. The velocity modulation coil is allowed to generate a magnetic field in the vertical direction to modulate a horizontal scanning velocity of the electron beams, whereby an edge of an image is enhanced (for example, see JP 57(1982)-45650 Y, JP 6(1994)-283113 A).
Furthermore, in a tube axis direction, a convergence and purity unit (CPU) is placed at a position overlapping the velocity modulation coil. The CPU is composed of dipole, quadrupole, and hexapole magnet rings, and a cylindrical holder provided externally on a neck of the funnel and holding these magnet rings. Each of the dipole, quadrupole, and hexapole magnet rings has a configuration in which two annular magnets are stacked. By adjusting the rotation angle around a tube axis of each magnet ring, the static convergence and purity of the electron beams are optimized.
A conductive film is applied to an inner wall surface of the funnel at a place where the deflection yoke is positioned, and is supplied with a high voltage by anode contact. Thus, when a power source of the color cathode-ray tube apparatus is turned ON/OFF, the above-mentioned substantially Ω-shaped metal band and metal screw, which fix the deflection yoke, are charged from the conductive film supplied with the above-mentioned high voltage, with the funnel and the insulating frame of the deflection yoke being dielectrics, and a discharge (spark) may occur toward the velocity modulation coil placed in the vicinity of the metal band and the metal screw. Such a discharge damages an electric circuit that drives the velocity modulation coil.
In order to prevent the occurrence of the discharge, for example, a method for grounding the above-mentioned substantially Ω-shaped metal band that fixes the deflection yoke through a lead to dissipate a charge is considered. However, according to this method, it is necessary to connect a lead, which increases the number of components and man-hours, resulting in an increase in a cost.
Furthermore, enlarging a distance in the tube axis direction between the metal band that fixes the deflection yoke and the velocity modulation coil so as to reduce the possibility of the occurrence of a discharge is considered. However, according to this method, the size of the color cathode-ray tube apparatus in the tube axis direction increases. Furthermore, generally, in terms of the enhancement of an image quality, it is considered to be advantageous that the position in the tube axis direction of an end on the phosphor screen side of the velocity modulation coil is as close as possible to the phosphor screen, and hence, the above-mentioned method contradicts this.
Japanese Utility Model Registration No. 3097458 describes that a removable disk-shaped barrier is provided at a holder of the CPU between the metal band and the velocity modulation coil. Japanese Utility Model Registration No. 3097458 describes the following: this barrier inhibits the formation of a discharge path from the metal band to the velocity modulation coil, so that a discharge can be prevented from occurring. Furthermore, Japanese Utility Model Registration No. 3097458 describes the following: by setting the barrier to be a member separate from the holder of the CPU, the barrier can be formed of a conductive resin with a low insulation resistance or metal; consequently, a discharge can be reduced further.
However, the barrier shown in Japanese Utility Model Registration No. 3097458 cannot prevent the occurrence of a discharge sufficiently. This will be described with reference to FIG. 7.
FIG. 7 is a vertical cross-sectional view showing a configuration around the CPU mounted on the neck of the color cathode-ray tube apparatus. This configuration is substantially symmetrical with respect to the tube axis, so that only one side with respect to the tube axis is shown in FIG. 7. Reference numeral 110 denotes a tube axis of a color cathode-ray tube, 120 denotes a neck of a funnel, 130 denotes a deflection yoke mounted on an outer circumferential surface of the funnel, 135 denotes an insulating frame of the defection yoke 130, 137 denotes a metal band that fixes the insulating frame 135 of the deflection yoke 130 to the neck 120, 140 denotes annular magnet rings constituting the CPU, 145 denotes a cylindrical holder holding the magnet rings 140, 150 denotes a velocity modulation coil fitted in grooves 148a, 148b of the holder 145, and 170 denotes a barrier engaged with the groove 148a of the holder 145. A metal screw that fastens both ends of the metal band 137 is not shown.
In the above configuration, when a charge amount accumulated in the deflection yoke 130 exceeds a certain value, a discharge path is formed, which extends from the metal band 137 to the velocity modulation coil 150 in the groove 148a, successively passing through an outer circumferential surface of the neck 120, a portion between the neck 120 and an inner circumferential surface of the barrier 170, and a portion between the barrier 170 and the holder 145. Thus, even if the barrier 170 is provided between the metal band 137 and the velocity modulation coil 150, a discharge path with a relatively short creepage distance is formed between the metal band 137 and the velocity modulation coil 150, so that the occurrence of a discharge cannot be prevented completely.
Furthermore, an operation of adjusting the rotation position around the tube axis of the magnet rings 140 of the CPU is performed while an image displayed actually on a screen is being watched. Thus, in the case of placing the barrier 170 for preventing the occurrence of a discharge between the metal band 137 and the velocity modulation coil 150, care should be taken so that the operability of the rotation adjustment of the magnet rings 140 of the CPU placed on an opposite side of the phosphor screen with respect to the barrier 170 is not impaired.